Enhanced thermoelectric performance of p-type Bi2Si2Te6 enabled via synergistically optimizing carrier concentration and suppressing bipolar effect

Abstract

<p>Bipolar diffusion is regarded as an obstacle to improving <a href="https://www.sciencedirect.com/topics/engineering/thermoelectrics" title="Learn more about thermoelectric from ScienceDirect's AI-generated Topic Pages">thermoelectric</a> performance because it dramatically deteriorates the thermopower and increases the <a href="https://www.sciencedirect.com/topics/physics-and-astronomy/thermal-conductivity" title="Learn more about thermal conductivity from ScienceDirect's AI-generated Topic Pages">thermal conductivity</a> at high temperatures. Here, we propose two effective strategies, either substitution of Bi with Mn, or replacement of Si with Ge, to optimize the carrier concentration and suppress the bipolar effect in Bi₂Si₂Te₆. The electrical resistivity is largely reduced and a compensation of the <a href="https://www.sciencedirect.com/topics/engineering/seebeck-coefficient" title="Learn more about Seebeck coefficient from ScienceDirect's AI-generated Topic Pages">Seebeck coefficient</a> by minority charge carriers is eliminated because of increased carrier concentration. Meanwhile, both Mn doping at Bi site and Ge alloying at Si site can achieve a larger DOS effective mass which is favorable to the <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/seebeck-effect" title="Learn more about Seebeck coefficient from ScienceDirect's AI-generated Topic Pages">Seebeck coefficient</a>. Accordingly, the maximum power factor is considerably improved from 4.4 μW cm⁻¹ K⁻² for pristine Bi₂Si₂Te₆ to 9.6 μW cm⁻¹ K⁻² for Bi_1.97Mn_0.03Si₂Te₆ and 9.2 μW cm⁻¹ K⁻² for Bi₂Si_1.8Ge_0.2Te₆. Furthermore, the lattice <a href="https://www.sciencedirect.com/topics/engineering/thermal-conductivity" title="Learn more about thermal conductivity from ScienceDirect's AI-generated Topic Pages">thermal conductivity</a> is largely diminished by ∼40% at elevated temperatures, which is ascribed to the suppressed bipolar <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/thermal-conductivity" title="Learn more about thermal conductivity from ScienceDirect's AI-generated Topic Pages">thermal conductivity</a> and strengthened <a href="https://www.sciencedirect.com/topics/physics-and-astronomy/phonon" title="Learn more about phonon from ScienceDirect's AI-generated Topic Pages">phonon</a> scattering by extra substitutional <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/point-defect" title="Learn more about point defects from ScienceDirect's AI-generated Topic Pages">point defects</a>. As a result, a peak <em>zT</em> value of unity is achieved in both Bi_1.98Mn_0.02Si₂Te₆ and Bi₂Si_1.8Ge_0.2Te₆ at 773 K.<br></p>